The present invention relates to a method of fabricating a print head in a serial printer, and more particularly to a method of fabricating a biasing plate spring used in a wire dot print head.
In known wire-dot print heads, armatures having print wires on their tips are attracted to cores by means of a magnetic flux from a permanent magnet. An opposing magnetic flux is generated by coils to cancel the magnetic flux from the permanent magnet, and printing is made by the drive of the wires.
Wire-dot print heads of this type use a biasing plate spring to give biasing forces when the armatures are released, have a good frequency response, and are called a spring charged wire-dot print head.
FIG. 1 is a cross sectional view of a spring-charged wire-dot print head.
It comprises print wires 1 for impacting printing medium through inked ribbon. The print wires 1 are bonded and fixed to tips of armatures Z. The armatures 2 are fixed to a biasing plate spring 8 and are supported in such a manner that they can swing with the biasing plate spring 3.
The biasing plate spring B is formed of a generally circular thin plate and has projections 4 toward the center. The armatures 2 are fixed to the projections 4.
The print head further comprises a first yoke 5. a second yoke 6, a first magnetic spacer 7, and a second magnetic spacer 8. The biasing plate spring 3 is held between the first magnetic spacer 7 and the second magnetic spacer 8. The print head further comprises a third yoke 9, a permanent magnet 10, and a core frame 11. The permanent magnet 10 generates a magnetic flux for attracting the armatures 2 to the core frame 11.
Demagnetizing coils 12 are wound on the core frame 11. When energized each coil 12 generates a magnetic flux opposing the magnetic flux of the permanent magnet 10, to release the armature 2 that has been attracted to the core frame 12.
An impact force adjustment screw 13 is provided to adjust the biasing force of the biasing plate spring 3. A middle guide 14 and a tip guide 15 are provided to guide the print wires. A head frame 10 is also provided.
When the coil 12 is not energized, the magnetic flux from the permanent magnet 10 passes through the third yoke 8, the first magnetic spacer 7, the second yoke 6, the first yoke 5, the armature 2, and the core frame 11, thereby forming a magnetic circuit. Owing to the magnetic attraction generated by the magnetic circuit, the armature 2 is attracted to the core frame 11, deforming the biasing plate spring 8.
When the coil 12 is energized, the magnetic flux generated by the coil 12 cancels the magnetic flux from the permanent magnet 10, to release the biasing plate spring 3 driving the print wire 1 fixed to the tip of the armature 2.
The biasing plate spring 3 used in the above wire-dot print head is formed by first producing an intermediate product 21 shown in FIG. 2 and FIG. 3. FIG. 2 is a plan view of the biasing plate spring intermediate product formed in the prior-art method of biasing plate spring fabrication, and FIG. 3 is a cross sectional view along line A-B in FIG. 2.
That is, metal spring material is first etched into an intermediate product 21 having an annular part 21a and a plurality of projections 4 extending inward from the inner periphery of the annular part 21a. Gaps 22 are formed between the projections 4.
The intermediate product 21 is then barrel-polished for removing flashes.
However, in the above-described prior-art method of fabrication of the biasing plate spring 3, the barrel-polishing process for removing flashes is conducted by placing several hundreds of biasing plate spring intermediate products 21 in a barrel machine, so, during the barrel-polishing process, one intermediate product may enter the gap 22 of another intermediate product 21.
As a result, the intermediate products damage each other, yielding defective products. When the defective products are used, with their defects undetected, they may break during use of the wire-dot print head.